Audit fixes + DVD support foundation (IFO, PS demux, MPEG-2, CSS crack)

Audit fixes (14 critical, 22 warnings):
- UDF: bounds checks on all ICB/FID parsing from disc data
- SCSI Linux: saturating_sub on residual, CDB length guard, buffer size guard
- SCSI macOS: SCSITaskStatus u32 (was u8 — stack corruption)
- AACS: EC mod_inv returns infinity instead of panic, key reduced mod n
- AACS: do_handshake tries all host certs (was returning on first failure)
- H.264: bounds check on SPS < 4 bytes
- ContentReader: error on missing unit key (was zero-fill)
- KEYDB: flat redirect loop (was recursive), 100MB response limit, Windows HOME fallback
- ISO writer: AVDP extent order, partition length, allocation cap
- Network: removed TCP_NODELAY on bulk stream
- MKV: guard on u64::MAX seek
- disc.rs: saturating_sub on extent offset, simplified dead region code
- cargo fmt (610 violations), cargo clippy --fix (55 auto-fixes)

DVD support (new files):
- src/ifo.rs — IFO parser (VIDEO_TS.IFO, VTS_XX_0.IFO, PGC chains, cells, streams) — 13 tests
- src/mux/ps.rs — MPEG-2 Program Stream demuxer (pack headers, PES, private stream 1) — 12 tests
- src/mux/codec/mpeg2.rs — MPEG-2 video parser (sequence headers, I-frame detection) — 15 tests
- src/css/crack.rs — split-attack algorithm (LFSR cipher needs verification — test ignored)

226 tests total (was 186), 1 ignored (CSS crack needs cipher verification).

src/mux/null.rs

@@ -1,8 +1,8 @@
 //! NullStream — discards all data. Write-only. For benchmarking.
 
-use std::io::{self, Read, Write};
 use super::IOStream;
 use crate::disc::DiscTitle;
+use std::io::{self, Read, Write};
 
 /// Null stream — accepts writes, discards data. For benchmarking rip speed.
 pub struct NullStream {
@@ -10,9 +10,18 @@ pub struct NullStream {
     bytes_written: u64,
 }
 
+impl Default for NullStream {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 impl NullStream {
     pub fn new() -> Self {
-        Self { disc_title: DiscTitle::empty(), bytes_written: 0 }
+        Self {
+            disc_title: DiscTitle::empty(),
+            bytes_written: 0,
+        }
     }
 
     pub fn meta(mut self, dt: &DiscTitle) -> Self {
@@ -20,12 +29,18 @@ impl NullStream {
         self
     }
 
-    pub fn bytes_written(&self) -> u64 { self.bytes_written }
+    pub fn bytes_written(&self) -> u64 {
+        self.bytes_written
+    }
 }
 
 impl IOStream for NullStream {
-    fn info(&self) -> &DiscTitle { &self.disc_title }
-    fn finish(&mut self) -> io::Result<()> { Ok(()) }
+    fn info(&self) -> &DiscTitle {
+        &self.disc_title
+    }
+    fn finish(&mut self) -> io::Result<()> {
+        Ok(())
+    }
 }
 
 impl Write for NullStream {
@@ -33,12 +48,17 @@ impl Write for NullStream {
         self.bytes_written += buf.len() as u64;
         Ok(buf.len())
     }
-    fn flush(&mut self) -> io::Result<()> { Ok(()) }
+    fn flush(&mut self) -> io::Result<()> {
+        Ok(())
+    }
 }
 
 impl Read for NullStream {
     fn read(&mut self, _buf: &mut [u8]) -> io::Result<usize> {
-        Err(io::Error::new(io::ErrorKind::Unsupported, "null stream is write-only"))
+        Err(io::Error::new(
+            io::ErrorKind::Unsupported,
+            "null stream is write-only",
+        ))
     }
 }